1. Field of the Invention
The present invention relates to a tape guide mechanism for a magnetic recording/reproducing apparatus having a cylindrical guide drum incorporating a rotary head.
2. Description of the Prior Art
FIG. 1 schematically shows a tape guide mechanism for a tape guide drum 1 incorporating a rotary head in a magnetic recording/reproducing apparatus of helical scan type.
The drum 1 is arranged inclined at a predetermined angle with respect to a reference surface 2. A magnetic tape 3 is passed from a guide roller 4 through a guide point a of an inclined guide pin 5 and is wrapped around the circumferential surface of the drum 1 so as to oppose a rotary head. The tape is wrapped through a wrap angle of about 180.degree. from an entrance point b to an exit point d through a central point c. The magnetic tape 3 is then passed to a guide roller 7 through a guide point e of an inclined guide pin 6. In FIG. 1, the respective points a, b, c, d and e are illustrated as points on the central line of the magnetic tape 3 along its widthwise direction. A tape lead step (not shown) having a lead angle .alpha. for regulating downward movement of the magnetic tape 3 is formed downward movement of themagnetic tape 3 is formed on the circumferential surface of the drum 1 in a known manner. The magnetic tape 3 is driven on this tape lead step.
In a tape guide mechanism of the type described above, the magnetic tape 3 can be stably driven by properly selecting, in accordance with a predetermined equation, the diameter and the inclination angle of the drum 1 with respect to the reference surface 2, the diameter and the inclination angle of the guide pins 5 and 6 with respect to the reference surface 2, the wrap angle of the magnetic tape 3 around the drum 1, i.e., the angle from the point b to the point d, a distance l.sub.1 between the points a and b, a distance l.sub.2 between the points d and e, and an angle .alpha. (lead angle) of the magnetic tape 3 at the point c with respect to a single plane f including the points a, c and e.
Other important conditions for stably driving the magnetic tape 3 are that the heights at the points a, c and e from the reference surface 2 be equal to each other, and the plane f be parallel to the reference surface 2.
In the above tape guide mechanism, the distances l.sub.1 and l.sub.2 are equal to each other. Therefore, the guide roller 4, the guide pin 5, the guide roller 7, and the guide pin 6 can be symmetrically arranged with respect to the drum 1. This allows easy design and reduces the overall size of the apparatus.
Thus, although the above-mentioned tape guide mechanism has various advantages as noted above, satisfactory regulation of widthwise movement of the magnetic tape 3 on the circumferential surface of the drum 1 cannot be obtained. For this reason, the magnetic tape 3 tends to float upward from the tape lead step on the circumferential surface of the drum 1.
In accordance with a technique proposed to overcome this drawback, when the magnetic tape 3 enters from the entrance point b, it is allowed to enter inclined upward by an elevation angle .theta..sub.1 as indicated by a dotted line in FIG. 1. When the magnetic tape 3 is passed through the exit point d, it is guided inclined upward at an elevation angle .theta..sub.2 as also indicated by a dotted line. A force acts to urge the magnetic tape 3 toward the tape lead step so as to regulate the widthwise movement of the magnetic tape 3.
This technique is disclosed, for example, in Japanese Patent Disclosure No. 58-121170. With the condition that the plane f is parallel to the reference surface 2, in order to incline the magnetic tape 3 at the angles .theta..sub.1 and .theta..sub.2 as indicated by the dotted lines in FIG. 1, the guide points a and e must be shifted with respect with respect to drum 1 to positions indicated by the dotted lines in FIG. 1. The That is, the guide pin 5 must be moved closer to the drum 1 and the guide pin 6 must be moved away from the drum 1 to positions indicated by the dotted lines. The angles .theta..sub.1 and .theta..sub.2 are normally small angles, e.g., 1.0 to 1.5.degree.. However, the corresponding moving distances of the guide pins 5 and 6 are relatively large. In this case, if the wrap angle of the magnetic tape 3 around the drum 1 is large, e.g., 180.degree. or more, no problem occurs. However, if the wrap angle is small, e.g., about 90.degree., the moving distance of the guide pin 6 is set to be several to several tens of times that of the guide pin 5. Thus, when the point a is moved on the plane f, if it is moved a small distance to the point a.sub.1 , the tape is inclined upward by the angle .theta..sub.1. However, when the point e is moved on the plane f, the magnetic tape 3 is not inclined upward by the angle .theta..sub.2 unless the point e is moved a long distance to the point e.sub.1.
As a result, as shown in FIGS. 2 and 3, the distance l.sub.2 is rendered far larger than the distance l.sub.1 (in the illustrated example, l.sub.2 .apprxeq.4l.sub.1). Then, the arrangement of the guide roller 4 and the guide pin 5, and the guide roller 7 and the guide 6 becomes nonsymmetrical with respect to the drum 1. A difficulty occurs in mechanism design, and prevents the manufacture of a compact apparatus.